Semiconductor devices

ABSTRACT

A CMOS circuit in low-voltage implementation, low power-consumption implementation, high-speed implementation, or small-size implementation. In a circuit which uses a FD-SOI MOST where a back gate is controlled by a well, voltage amplitude at the well is made larger than input-voltage amplitude at the gate. Alternatively, the circuit is modified into a circuit which uses a MOST that changes dynamically into an enhancement mode and a depletion mode.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation of U.S. application No. 11/362,172filed on Feb. 27, 2006. Priority is claimed based on U.S. applicationNo. 11/362,172 filed on Feb. 27, 2006, which claims the priority ofJapanese Patent No. 2005-201054 filed Jul. 11, 2005, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device in which CMOScircuits are integrated on a semiconductor chip. More specifically, itrelates to a circuit scheme and a device scheme which allow a reductionin power consumption, operating voltage, or area of a CMOS integratedcircuit device.

DESCRIPTION OF THE RELATED ART

As is well known, in a MOS transistor (MOST), the operating voltageV_(DD) decreases as the patterning of the MOST becomes finer. Thisresults in an increase in the variation in threshold voltage (V_(T)) ofthe MOST. For example, FIG. 10A illustrates the relationship betweenV_(DD)-V_(TO) and fine-patterning process technology. Here,V_(DD)-V_(TO) denotes effective gate voltage (i.e., gate-over drivevoltage) of the MOST, and V_(TO) denotes average V_(T) of the MOST. Theexpected values according to ITRS 2001 (: International TechnologyRoadmap for Semiconductors, 2001 ed.) are added to FIG. 10A. In thisdrawing, MPU and LPSOC denote a microprocessor and a low-power System Ona Chip, respectively. Here, a lower-limit exists to this V_(TO). By theway, sub-threshold current (i.e., a kind of leakage current) which flowsin the MOST at non-conduction time increases exponentially inaccompaniment with a decrease in V_(T). In order to suppress thisexponential increase, the above-described lower-limit is given. Thislower-limit value, which is substantially irrelevant to the finepatterning of the MOST, is determined by an upper-limit value of theleakage current on the chip. Usually, this lower-limit value is equal toan order of 0.2 V to 0.3 V in the case of a high-speed chip, and anorder of 0.5 V in the case of a low-power chip. Accordingly, asillustrated in the drawing, the effective gate voltage quickly lowers inaccompaniment with the fine patterning. This results in a rapid loweringin operation speed of MOSTs within a load-driving circuit. This isbecause the operation speed of a MOST is substantially inverseproportional to the effective gate voltage. For example, in the case ofthe high-speed chip, V_(DD) lowers in accompaniment with the finepatterning, thus getting closer to 0.3 V. At this time, the circuitspeed is delayed rapidly.

The increase in the variation (ΔV_(T)) in the threshold voltage V_(T) inaccompaniment with the finer patterning is also a serious problem. Thisis because the operation speed of a MOST within a chip is becomingincreasingly varied, thereby making the reliable chip design difficultto accomplish. FIG. 10B illustrates a drawing issued in M. Yamaoka etal., “Low Power SRAM Menu for SOC Application Using Yin-Yang-FeedbackMemory Cell Technology”, Symp. VLSI Circuits Dig., pp. 288 to 291, Jun.2004. As illustrated in the drawing, standard deviation a of thevariation in the threshold voltage V_(T) increases in accompaniment withfiner patterning of the MOST. In the drawing, σ_(int) denotes standarddeviation of the so-called intrinsic V_(T) which is determined by avariation in the number of impurity atoms within channel of the MOST ora variation in the position thereof, while σ_(ext) denotes standarddeviation of the so-called extrinsic V_(T) which is determined by avariation in size of the channel or the like. The entire V_(T) variationσ is determined by both of the variations. Even in the case where thefine-patterning technology employed is of an order of 90 nm, σ attainsto as much as an order of 30 mV. The design of a single chip needs to beperformed by taking into consideration the V_(T) variation (ΔV_(T)) ofan order of 5σ, and this value attains to as much as 150 mV. Under thisvariation, the effective gate voltage of each MOST within the singlechip, which is represented by V_(DD)−(V_(TO)+ΔV_(T)), turns out to besignificantly varied. For example, in the case where V_(TO)=0.3 V andΔV_(T)=150 mV, when V_(DD) becomes equal to 0.45 V, driving current foreach MOST becomes equal to 0, and thus the circuit speed diverges toinfinity.

Conventionally, two methods have been proposed in order to suppress theabove-described occurrences of the low operation-speed and theoperation-speed variation caused by the finer patterning and thelow-voltage implementation. One method is development of a MOST which iscapable of making ΔV_(T) smaller. The other method is development of acircuit which allows V_(T) to be dynamically changed depending on theoperation state. Namely, this circuit is a one which, at OFF time,maintains V_(T) at a constant magnitude in order to suppress thesub-threshold current, and which, at ON time, makes V_(T) smaller inorder to make the effective gate voltage larger. As will be explainedbelow, the use of a fully-depleted double-gate SOI (:Silicon-On-Insulator) (hereinafter, referred to as “FD-SOI”) MOST makesit possible to simultaneously satisfy these two conditions. The reasonsfor this are as follows: Namely, as indicated in FIG. 10B, the FD-SOIstructure itself allows reductions in the variation in the thresholdvoltage V_(T). Moreover, taking advantage of this FD-SOI structure in acircuit-mannered configuration makes it possible to implement thedynamic V_(T) as is described above.

The detailed structure and characteristics of the FD-SOI MOST aredescribed in R. Tsuchiya et al., “Silicon on Thin Box: A New Paradigm ofThe CMOSFET for Low-Power and High-Performance Application FeaturingWide-Range Back-Bias Control”, IEDM Dig. Tech. Papers. pp. 631-634, Dec.2004. The details of this structure will be explained hereinafter. FIG.11A, FIG. 11B, and FIG. 11C are an A-A cross-sectional view and a planview of an N-channel MOST (NMOST) and a P-channel MOST (PMOST), and aB-B cross-sectional view of the NMOST, respectively. Also, FIG. 12A andFIG. 12B illustrate an equivalent circuit to the NMOST and a one to thePMOST, respectively. Incidentally, FIG. 11C indicates an example wherethe gate and the well are connected to each other. The referencenumerals denote the following configuration components: 20 denotes ametal-silicide (such as NiSi) gate electrode, 3 denotes a single-crystalsemiconductor thin film (i.e., SOI layer), 13 and 14 denotejunction-capacitance-reducing n-type impurity diffusion layers, 8denotes a high-concentration n-type impurity ultra-thin source diffusionlayer, 9 denotes a high-concentration n-type impurity ultra-thin draindiffusion layer, 4 denotes a BOX (: Buried OXide) layer, 25 and 26denote threshold-voltage control diffusion layers (i.e., well layers),and the like. The feature of this MOST is as follows: Namely, thethreshold voltage V_(T) can be controlled based on type of the gatematerial, concentration of the well layer under the BOX layer, andvoltage to be applied to the well layer. In the actual MOST, channellength (Lg) is equal to 100 nm or less, the gate material is, e.g., themetal silicide such as nickel silicide (NiSi), thickness of the SOIlayer which allows the MOST to be formed is equal to 20 nm or less,thickness of the BOX layer is equal to 10 nm or less, the concentrationof the well layer located under the BOX layer is equal to an order of10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. As illustrated in FIG. 10B, the employment ofthe thin BOX film or the like makes it possible to reduce the variationin V_(T) of the FD-SOI MOST down to 20% or less in the conventional bulkstructure.

The above-described double-gate MOST structure can be regarded as asingle MOST which results from connecting an upper MOST and a lower MOSTin parallel. Here, in the lower MOST, the well layer becomes a gate, andthe BOX layer becomes a gate insulating film. Accordingly, changing thewell voltage in the lower MOST makes it possible to significantly changethe entire threshold voltage V_(T) of the double-gate MOST. This isbecause the well layer is insulated from the other components and thusthe well voltage can be significantly changed without causingpn-junction leakage current to occur. FIG. 13A and FIG. 13B illustrate aconventional circuit which utilizes this characteristic. For example, inthe well-known CMOS inverter illustrated in FIG. 13A, the gates and thewells are directly connected to each other as is illustrated in FIG.13B. This direct connection allows V_(T) to be dynamically changeddepending on the circuit input, i.e., the gate voltage V_(DD) of theMOST. Namely, in the NMOST, at the input voltage at which the NMOST isswitched OFF (i.e., 0 V), the well voltage becomes equal to 0 V, whichis small. As a result, V_(T) becomes larger. Meanwhile, at the inputvoltage at which the NMOST is switched ON (i.e., V_(DD)), the wellvoltage becomes higher. As a result, V_(T) becomes smaller. Here, V_(T)is selected in advance as being large enough so that the sub-thresholdcurrent at the OFF time will become smaller than a tolerance value.Then, at the ON time, the effective gate voltage will become larger bythe amount by which V_(T) has become smaller. This allows implementationof the high-speed operation of the NMOST. An example where the MOSTconnected in this way is applied to a SRAM (: Static Random-AccessMemory) cell is disclosed in M. Yamaoka et al., “Dynamic-Vt,Dual-Power-Supply SRAM Cell using D2G-SOI for Low-Power SoCApplication”, Int'l SOI Conf. Dig. Tech. Papers. pp. 109-111, Oct. 2004.

SUMMARY OF THE INVENTION

-   (1) In the conventional circuit, lowering the V_(DD) results in a    lowering in the effect of the dynamic V_(T). The reasons for this    are as follows: Namely, since the gate and the well are directly    connected to each other, if the input voltage V_(DD) becomes    smaller, the change in the well voltage also becomes smaller, and    thus the change in V_(T) becomes smaller. Moreover, since the    circuit is operated by the single voltage V_(DD), it is impossible    to use another voltage which would compensate for adverse influences    caused by the lowering in V_(DD).-   (2) In the conventional circuit as well, the proposals up to the    present have been limited to the application to the SRAM cell. The    reason for this is as follows: Namely, in the circuit where the    gates and the wells are directly connected to each other, factors    such as the operation conditions and a limit to the low-voltage    implementation have been not clarified in relation to the device    characteristics of the FD-SOI MOST.

It is an object of the present invention to clarify the relationstherebetween, and, based on this clarification, to provide effectivedevice structures and circuit operation conditions, or novel low-voltagecircuits.

According to the present invention, voltage amplitude at a well is madelarger than input-voltage amplitude at a gate. Alternatively, thevoltage amplitude at the well is made larger than voltage amplitude at adrain or voltage amplitude at a source. Alternatively, voltageamplitudes at wells within a plurality of circuits are set at mutuallydifferent values. Alternatively, the circuit is operated by multivoltages. Alternatively, the circuit is modified into a circuit whichuses MOSTs that change dynamically into an enhancement mode and adepletion mode. Alternatively, there is provided a new circuit which, atinput stage of a differential input circuit, uses a MOST where the gateand the well are directly connected to each other.

According to the present invention, it becomes possible to accomplishlow-voltage implementation, low power-consumption implementation,high-speed implementation, or small-size implementation of a CMOScircuit.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a CMOS inverter to which the presentinvention is applied;

FIG. 2 is a characteristics diagram of transistors used in the presentinvention, where L_(g), t_(SOI), t_(BOX), and N_(sub) denote gate lengthof the MOSTs, film thickness of a SOI layer, film thickness of a BOXlayer, and impurity concentration of a well layer under the BOX layer,respectively;

FIG. 3 is a schematic diagram of a voltage converter;

FIG. 4A, FIG. 4B, and FIG. 4C illustrate application embodiments of thepresent invention to power-supply switches, respectively;

FIG. 5A and FIG. 5B illustrate application embodiments of the presentinvention to a repetition-circuit block, respectively;

FIG. 6A and FIG. 6B illustrate embodiments of power-supply switches ofthe present invention, respectively;

FIG. 7A and FIG. 7B illustrate differential amplifiers to which thepresent invention is applied, respectively;

FIG. 8 illustrates an application embodiment of the present invention toa DRAM sense system;

FIG. 9 illustrates an embodiment of the CMOS inverter of the presentinvention and a level conversion circuit;

FIG. 10A and FIG. 10B illustrate the lowering trend of the effectivegate voltage, and the comparison in the variations (90 nm, 65 nm, 45 nm,and 32 nm denote the device sizes) in the threshold voltage between theconventional bulk CMOS transistor (bulk at the upper stage) and theconventional FD-SOI MOS transistor (FD-SOI at the lower stage) used inthe present invention, respectively;

FIG. 11A, FIG. 11B, and FIG. 11C are the cross-sectional views and theplan view of a conventional FD-SOI MOS transistor;

FIG. 12A and FIG. 12B illustrate a equivalent circuits to a conventionalFD-SOI MOS transistor; and

FIG. 13A and FIG. 13B illustrate a conventional CMOS inverter.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, referring to the drawings, the explanation will be givenbelow concerning the present invention.

Embodiment 1

FIG. 1 illustrates a CMOS inverter circuit which is a first embodimentof the present invention. A first circuit includes, e.g., an inverterINV, and a second circuit includes, e.g., a converter CNV forcontrolling well voltages with large voltage amplitudes. The secondcircuit is a sub-circuit. This sub-circuit drives comparatively smallwell capacities of MOSTs within the first circuit, and thus is easy todesign into small-size, high-speed, and low power-consumptionimplementation. In contrast thereto, the first circuit is amain-circuit. This main-circuit must generically drive large loadcapacities, and thus cannot help being formed into large-area,low-speed, and large power-consumption implementation. Consequently,adding the second circuit to the first one results in exertion of only asmall influence on the entire area, speed, and power consumption.

A PMOST (M_(P1)) and an NMOST (M_(N1)) are the FD-SOI MOST illustratedin FIG. 11 and FIG. 12. FIG. 2 illustrates the relationship of thethreshold voltages V_(T) of these MOSTs relative to the well voltages.Here, V_(T) are the values defined using a constant current (1 nA/μm).In the NMOST, if V_(T) has a positive sign, the NMOST assumes anenhancement mode. Accordingly, the NMOST is switched OFF more deeply asV_(T) becomes larger. If V_(T) has a negative sign, the NMOST assumes adepletion mode. Consequently, the current will flow even if the gatevoltage is equal to 0 V, and a larger driving current can be flown asabsolute value of V_(T) becomes larger. In the PMOST, the polaritybecomes inverted to that of the NMOST. The threshold voltages V_(T) aremeasured in compliance with ordinary conditions, and V_(T) of the NMOSTmeasured when the well voltage is set at 0 V is equal to 0.2 V, andV_(T) of the PMOST measured when the well voltage is set at V_(DD) isalso equal to substantially 0.2 V. The converter CNV converts a pulse,which is inputted into an input terminal IN and whose amplitude is equalto V_(DD), into a same-polarity pulse which is provided with a largeramplitude ranging from V_(BB) to V_(DH). Then, the converter CNV appliesthis same-polarity and larger-amplitude pulse to the wells of M_(p1) andM_(N1). On account of this, between the threshold voltages V_(T) ofM_(p1) and M_(N1), one voltage becomes larger and the other voltagebecomes smaller, and vise versa. For example, setting V_(DD)=0.5 V,V_(BB)=−0.5 V, and V_(DH)=1 V, assume that a pulse which will changefrom 0 V to 0.5 V is inputted. Since the well voltage of the NMOST ismade larger and becomes equal to V_(DH), V_(T) changes to a point d inthe drawing where V_(T) is smaller. At this point, the NMOST changesinto a depletion mode where V_(T)=−0.2 V. Accordingly, the NMOST findsit possible to drive (i.e., discharge) load capacity of an outputterminal OUT at high speed and with a large current. At this time, thePMOST is switched OFF. This is because, since the well voltage of thePMOST also becomes equal to V_(DH), its V_(T) changes to a point d′ inthe drawing, and thus the PMOST changes into an enhancement mode whereV_(T) is larger. On the other hand, if the input pulse changes fromV_(DD) to 0 V, the output of the converter changes from V_(DH) toV_(BB). Accordingly, the NMOST changes from the point d to a point c,thus changing into an enhancement mode where V_(T) is substantiallyequal to 0.3 V. The PMOST transitions from the point d′ to a point c′,thus changing into a depletion mode where V_(T) is substantially equalto 0.1 V. Consequently, this time, the PMOST will drive (i.e., charge)the load capacity of the output terminal OUT at high speed and with alarge current that the PMOST has generated.

As is obvious from the above-described explanation, whenever the inputis changed, either of the NMOST and the PMOST becomes conductive, andthe other is maintained at the high V_(T) (e.g., 0.2 V or more) at whichthe sub-threshold current is tolerable. This configuration allowsimplementation of a completely novel circuit, i.e., the dynamic-modeenhancement/depletion CMOS inverter where, depending on the input, thethreshold voltages V_(T) dynamically change between the enhancement modeand the depletion mode. Unlike the conventional CMOS inverterillustrated in FIG. 13B, this effect results from making the wellvoltage amplitudes of the in-inverter MOSTs larger than the gateinput-voltage amplitude of the in-inverter MOSTS, or making the wellvoltage amplitudes of the in-inverter MOSTs larger than the drainvoltage amplitudes or source voltage amplitudes of the in-inverterMOSTS. Incidentally, as is publicly known, V_(DH) and V_(BB) can begenerated based on the power-supply voltage V_(DD) and using acharge-pump circuit, for example.

Hereinafter, in order to clarify differences in the effect between theCMOS inverter according to the present invention and a conventional CMOSinverter (FIG. 13B), the more detailed explanation will be given below.

In the process where the input waveform is changing from 0 to V_(DD),the threshold voltages V_(T) of the MOSTs change dynamically with theintervention of the converter CNV. Now, take the NMOST (M_(N1)) as theexample. Here, conversion ratio of the input voltage (V_(IN)) into thewell voltage (V_(well)) is denoted by k₁, and change ratio of V_(T) inresponse to the change in the well voltage is denoted by k₂. Also,assume that V_(T) at the time when the input voltage is equal to 0 V,i.e., V_(T) (0), is equal to 0.2 V (corresponding to the point a in thedrawing). This value is large enough to make the sub-threshold currenttolerable. When the input voltage changes from 0 V to V_(IN), V_(T) ofthe NMOST becomes equal to V_(T) (0) −k₂V_(well). If, at this time, theNMOST is switched ON, the following expressions hold:V_(well) =k ₁V_(IN)V_(IN)=V_(T)(0)−k ₂V_(well)∴V_(IN)=V_(T)(0)/(1+k ₁ k ₂).  (1)

Accordingly, the input voltage at which the NMOST starts to be switchedON becomes smaller by the amount of coefficient (1+k₁k₂). Actually, thisvery input voltage, which is none other than V_(T) of the NMOSTgenerated as the result of the dynamic change, indicates that the inputvoltage has become smaller.

Then, using this expression, with respect to the input voltage at whichthe NMOST starts to be switched ON, let's make the comparison among thefollowing cases: Namely, the case where the well voltage is fixed as isillustrated in FIG. 13A, the case where the gate and the well aredirectly connected to each other as is illustrated in FIG. 13B, and thecase where, by the converter, the input voltage is converted into alarger voltage, and this larger voltage is applied to the well as isillustrated in FIG. 1. In the case where the well voltage is fixed,V_(IN)=0.2 V holds since k₁=0 and k₂=0 are established, and thus V_(T)remains static and unchanged. In the case where the gate and the wellare directly connected to each other, k₁=1 holds. Also, k₂=0.4 isestablished on average. This is because, when focusing attention on therelation ranging from the point a to the point d, V_(T) changes by 0.4 Vas the well voltage changes by 1 V. Accordingly, from the expression(1), the NMOST starts to be switched ON at V_(IN)=0.14 V. In the case ofthe converter scheme, k₁=2 is established since V_(DD) (=0.5 V) isconverted into V_(DH) (=1 V). Consequently, from the expression (1), theNMOST starts to be switched ON at V_(IN)=0.11 V. Here, in theapplication to mobile appliances or the like where requirements for theleakage current are severe, V_(T) (0)=0.5 V holds. In this case as well,the input voltage at which the NMOST starts to be switched ON can bedetermined from the expression (1) in basically the same way. Namely,the NMOST is permitted to operate at the input voltages of the followingascending order, i.e., 0.28 V in the converter intervention scheme, 0.36V in the gate-well direct connection scheme, and 0.5 V in thewell-voltage fixed scheme.

Obviously, it is preferable that the change ratio k₂ of V_(T) relativeto the well voltage be larger in order to enhance the effect of thedynamic V_(T) like this.

Incidentally, the following setting is preferable in order to change theNMOST into the depletion mode thereby to enhance the dynamic V_(T)effect:V_(T)(0)−k ₂V_(well=V) _(T)(0)−k ₁ k ₂V_(IN<)0.  (2)

Consequently, a circuit scheme which will make k₁ larger is of firstimportance. Implementation of this circuit scheme requires that thewell-voltage amplitude be made as large as possible. The embodiment inFIG. 1 satisfies this very requirement. Alternatively, in the case ofthe circuit in FIG. 13B, using larger input voltage, i.e., largerV_(DD), is effective. A MOST structure which will make k₂ larger is ofanother importance. Implementation of this MOST structure requires thatthe thickness of the BOX layer be made thinner to enhance functionalityof the under-BOX MOST. Making the thickness too thin, however, causestunnel current to occur. Accordingly, an order of 2 nm insilicon-dioxide film thickness is its limit. Here, in addition to thesilicon-dioxide film of 2-nm-thick order, films such as oxynitride(SiON) film are also preferable. This is because the gate-film interfacemaintains stability even at a high-temperature processing temperatureneeded after formation of the BOX layer. In this case, the BOX layer canbe made thin down to an order of 1.5 nm in equivalent silicon-dioxidethickness. The operation and effect explained so far are also basicallythe same in the case of the PMOST.

In the dynamic-V_(T) scheme, after the MOSTs start to be switched ON inthe process where the input waveform is starting up, the thresholdvoltages V_(T) of the MOSTs are gradually becoming smaller inaccompaniment with the increase in the input voltage. As a result, bythe amount equivalent thereto, influence of the V_(T) variation exertedon the driving current, i.e., the influence of the V_(T) variationexerted on the operation-speed variation, becomes relatively smaller ascompared with the fixed V_(T). Consequently, the dual effectsimplemented by the FD-SOI MOST, i.e., the V_(T)-variation reductioneffect implemented by the FD-SOI MOST itself indicated in FIG. 10B andthe V_(T)-variation reduction effect implemented by the dynamic-V_(T)circuit scheme, make it possible to neglect the influence of the V_(T)variation exerted on the operation-speed variation.

In the embodiment in FIG. 1, if the second circuit (converter CNV) ishighly-sensitive and highly-speedy enough to detect an infinitesimalchange in the input voltage, inputting even an extremely low V_(DD)enables the first circuit (inverter INV) to be operated at high speed.For example, as explained earlier, in the application to mobileappliances or the like where V_(T) (0)=0.5 V is requested, the minimumvoltage which permits the MOSTs within the first circuit to operate isequal to 0.5 V in the well-voltage fixed scheme, 0.36 V in the gate-welldirect connection scheme, and 0.28 V in the converter scheme. This showsthat, even if V_(DD) has lowered down to an order of 3 V, the converterscheme permits the operation although the other two schemes do notpermit the operation. This is because the converter scheme lowers V_(T),and thus allows implementation of the high sensitivity. Accordingly,high-sensitivity design for the converter becomes important.Incidentally, in addition to the above-described voltage settings,combination of V_(BB)=0 and V_(DH)>V_(DD), or combination ofV_(DH)=V_(DD) and V_(BB)<0 (namely, V_(BB) is negative) is possibledepending on conveniences for the design.

FIG. 3 illustrates a concrete example of the high-sensitivity andhigh-speed converter (CNV). In order to make smaller V_(T) of inputMOSTs (i.e., M_(N21), M_(P21)) of the converter, circuits where thegates and the wells are directly connected to each other are employedthereby to allow implementation of the high sensitivity. Moreover, afterthe input voltage has been detected by these input MOSTs with the highsensitivity, the input voltage detected is converted into a largervoltage amplitude by feedback MOST circuits (i.e., M_(P22), M_(P23),M_(N22), M_(N23)) which are characteristic of high-speed operations. Inthese circuits, since the gates and the wells are directly connected toeach other as is the case with the conventional circuit in FIG. 13B,there exists the characteristic that the threshold voltages V_(T) changedynamically. Unlike the conventional circuit, however, the wells andcircuits of M_(P22), M_(P23), M_(N22), and M_(N23) are operated withlarger voltage amplitudes which differ from voltage amplitude at thewells of the MOSTs (M_(N21), M_(P21)) within the input circuit. As aresult, variations in the threshold voltages V_(T) of the feedback MOSTsare larger by the amount equivalent thereto. This results in anadvantage that the feedback effect is made even larger. For example, ifM_(N21) is switched ON at the input voltage smaller than V_(DD), gatevoltage of M_(P23) lowers, and thus an output OUT' starts to be chargedup to V_(DH). Since M_(P22) is driven into the OFF state inaccompaniment therewith, the gate voltage of M_(P23) lowers evenfurther. Eventually, the output will be charged to V_(DH) at high speedby the tremendous feedback effect implemented by these larger voltageamplitudes. Of course, it is one of the causes for the high-speedimplementation that capacities of the wells of these feedback MOSTs aresmaller than the load capacity of the first circuit. On the other hand,if the input voltage changes from V_(DD) to 0 V, M_(N21) is switchedOFF, and M_(P21) is switched ON. Accordingly, the output will bedischarged down to V_(BB) at high speed in basically the same way.

Here, referring to FIG. 2 and focusing attention on the variations inthe threshold voltages V_(T), the more concrete explanation will begiven below concerning the operation of the converter circuit in FIG. 3.In M_(N21), V_(T) changes in the range of the point a to the point b,and, in M_(P21), V_(T) changes in the range of the point a′ to the pointb′. Accordingly, the variations in V_(T) are smaller by the amount ofthe smaller change in the gate input voltage. Meanwhile, in M_(P22) andM_(P23), V_(T) change in the range of the point c′ to the point d′, and,in M_(N22) and M_(N23), V_(T) change in the range of the point c to thepoint d. Consequently, the variations in V_(T) are larger by the amountof the larger change in the gate input voltage, i.e., the voltage changemade from V_(DH) to V_(BB). Obviously, as regards the values of V_(T) atthe OFF time, the input MOSTs (M_(N21), M_(P21)) are smaller than thefeedback MOST circuits (M_(P22) and M_(P23), M_(N22) and M_(N23)) by theamount of the differences in the voltage amplitudes.

Embodiment 2

FIG. 4A, FIG. 4B, and FIG. 4C illustrate power-supply switches to whicha MOST whose well is driven with a large voltage amplitude is applied,and which are then made small-sized. Here, a first circuit is apower-supply switch, and a second circuit is an internal core circuit(CORE). The operating voltage for the internal core circuit is small,and accordingly the threshold voltage V_(T) of the MOST is also small.This results in flowing of a large sub-threshold current therein. FIG.4A illustrates the following circuit: Using the PMOST (M_(p)),power-supply V_(DD) for the internal core circuit is cut off from theinternal core circuit during an unnecessary time-zone, thereby cuttingthe sub-threshold current in the internal core circuit. During theunnecessary time-zone, e.g., a time-period such as standby time or sleepmode, switching the PMOST completely OFF requires that V_(T) of thePMOST be made larger. On the other hand, during a time-zone in which theinternal core circuit operates, feeding an enough ON-current to theinternal core circuit requires that V_(T) of the PMOST be made smallenough. This is because, otherwise (i.e., if V_(T) were not made smallenough), the existence of the switch PMOST would exert bad influences onthe operation of the internal core circuit. In the present embodiment,unlike the conventional circuit in FIG. 13B, the characteristic is asfollows: Namely, the well voltage amplitude of the switch MOST is madelarger than the gate input-voltage amplitude of the switch MOST, or thewell voltage amplitude of the switch MOST is made larger than the drainvoltage amplitude or source voltage amplitude of the switch MOST.

In a microprocessor chip or the like, in order to feed the enoughcurrent to the internal core circuit, channel width of the switch PMOSTattains to 3 m or more. This, in the conventional switch MOST, increasesthe gate capacity as well as the area. Consequently, there exists adanger of making it difficult to design a circuit for controlling thegate voltage. If V_(T) at the ON time of the MOST is made smaller, thecurrent feeding capability will be enhanced. The threshold voltagesV_(T), however, is the value which is always fixed. This condition makesV_(T) at the OFF time smaller as well, thereby making it impossible toshield the MOST. As a result, the sub-threshold current in the MOSTbecomes a problem. In the present embodiment, however, V_(T) of the MOSTcan be made larger at the OFF time. Accordingly, it becomes possible toshield the MOST completely. Also, V_(T) of the MOST can be made smallerat the ON time. Consequently, it becomes possible to feed the enoughcurrent to the internal core circuit even if the channel width of theMOST is narrower. Since the internal core circuit generally occupies alarge area, an area increase by the converter CNV is negligible. FIG. 4Billustrates an embodiment where the NMOST (M_(N)) is used as the switchMOST. This embodiment is expected to exhibit basically the same effect.FIG. 4C illustrates an embodiment where the switches are used on both ofthe power-supply side and the ground side. If the output from theconverter CNV is inverted by an inverter and is applied to M_(N),setting up only one converter will suffice.

Embodiment 3

FIG. 5A illustrates an embodiment where the present invention is appliedto a MOST switch (i.e., first circuit) and a repetition-circuit block(BLK) (i.e., second circuit). Here, this repetition-circuit block is,e.g., a word-driver block of memory. Each MOST included therein is theFD-SOI MOST. The respective sources of output PMOSTs (M_(W)) ofinverters are collectively connected to a single common source line PWL.Moreover, the PWL is connected to power-supply voltage V_(DD) via theswitch MOST (M_(p)). At the time of selection for an inverter, theswitch MOST (M_(p)) is switched ON, and the input voltage into oneinverter INV out of the n inverters within the block BLK is made equalto 0 V. This operation selects the one inverter INV, thus selecting anddriving the corresponding one word line from among n word lines (i.e.,WL₀, . . . , WL_(n-1)). On the other hand, at the time of thenon-selection, the gate voltage of M_(p) is made equal to V_(DD) toswitch M_(p) OFF. Also, the input voltages into all the inverters aremade equal to V_(DD) to switch all the inverters OFF. Here, the switchMOST (M_(p)) is added to the block BLK in order to reduce asub-threshold current which will flow in the output PMOSTs (M_(W))within the block BLK. By the way, the following fact has been wellknown: Namely, for simplicity, assuming that the threshold voltagesV_(T) of all the MOSTs are equal to each other at the time of thenon-selection, the sub-threshold current will be reduced down toW_(p)/nW by the switch MOST (M_(p)). Here, W_(p) and W denote channelwidth of the switch MOST (M_(p)) and that of the output PMOSTs (M_(W))of the inverters. Here, considering the above-described condition at thetime of the selection shows that a certain relationship holds betweenW_(p) and W. Namely, at the time of the selection, the switch MOST(M_(p)) is switched ON, and only one inverter within the BLK is driven.As a result, as long as both of them (i.e., M_(p) and the output-stagePMOST within the only one inverter) have the equal V_(T), selectingW_(p) as being of an order of 10W allows the corresponding word line tobe driven with the driving speed scarcely dropped even if the switchMOST is added. Accordingly, at the time of the non-selection, thesub-threshold current will be reduced down to an order of 10/n. As aresult, the larger n is, i.e., the larger the number of the word linesis, the more the sub-threshold current will be reduced. Here, at thetime of the selection, the use of the converter allows V_(T) of theswitch MOST to be made small enough or, as described earlier, permitsthe switch MOST to change into the depletion mode. This makes itpossible to make W_(p) small enough down to 10W or less, whichcorresponds to the amount equivalent to the driving current capable ofbeing taken then. Consequently, the switch MOST can be made small-sized.FIG. 5B illustrates another embodiment. Here, the following fact hasbeen well known: Namely, one block is divided into a large number ofsub-blocks, then selecting a sub-block which is wished to be selected.This allows implementation of the low power consumption when seen as awhole. The present embodiment results from adding, to the respectivesub-blocks, the switch in FIG. 5A which is equipped with the selectionfunction. Only a switch MOST corresponding to a sub-block wished to beselected is switched ON. As compared with the embodiment in FIG. 5A,size of each switch MOST can be made smaller by the amount equivalent tothe implementation of the division. Accordingly, well capacity of eachswitch MOST can be made smaller. Consequently, it is effective enough toselect and drive a single switch MOST alone which has the small capacitylike this. This allows implementation of the low power consumption.

Incidentally, the embodiments illustrated in FIG. 4A, FIG. 4B, FIG. 4C,FIG. 5A, and FIG. 5B are the ones where the power-supply switchillustrated in FIG. 6A is used. Also, as illustrated in FIG. 6B, apower-supply switch is also usable where the gate and the well aredirectly connected to each other, and where a pulse whose amplitude ismade equal to V_(DH) and V_(BB) is applied to this gate. Theimplementation of this large amplitude makes it possible to make thechange in V_(T) larger. Also, as will be explained in FIG. 8, whenperforming in-batch driving of a large number of switch MOSTssimultaneously, the converter CNV for driving each MOST can be omitted.This allows implementation of the smaller area as a whole.

Embodiment 4

The inventor et al. have noticed for the first time that theconventional circuit in FIG. 13B, depending on its application, willexhibit an amplification effect. FIG. 7A and FIG. 7B illustrateamplifiers which result from taking advantage of this amplificationeffect exhibited by the conventional circuit. FIG. 7A illustrates alatch sense amplifier used in such devices as a DRAM (: DynamicRandom-Access Memory). Here, the gates and the wells are directlyconnected to each other. This makes the input voltage effectivelylarger; that is to say, the amplification effect exists effectively. Forexample, take the following case as an example: One (in1) of inputs isequal to V_(DD)/2−V_(s), which results from superimposing a signalvoltage v_(s) on a floating voltage V_(DD)/2, and the other input (in2)is equal to the floating voltage V_(DD)/2. Accordingly, well voltages ofthe NMOSTs (i.e., M_(N1) and M_(N2)) are also equal to V_(DD)/2 andV_(DD)/2-v_(s), respectively. As is apparent from the drawing, V_(T) ofM_(N2) becomes larger by δV_(T) by the negative signal voltage v_(s).Namely, by the amount of δV_(T), M_(N2) finds it more difficult to beswitched ON. Conversely, by this amount of δV_(T), M_(N1) finds iteasier to be switched ON. This is because the signal voltage becomeseffectively larger by the amount of δV_(T). Consequently, if anactivation pulse is inputted into a common terminal/ACT, M_(N1) becomeseasier to be conductive, and thus the input (in1) will be dischargeddown to 0 V at high speed. In this process, M_(N2) is becoming more andmore difficult to be conductive, and thus the input (in2) will stop tobe discharged at a certain constant voltage. FIG. 7B illustrates awell-known current-mirror amplifier frequently used in such devices as aSRAM. This current-mirror amplifier also effectively exhibits theamplification effect in basically the same way. Here, V_(REF) denotesreference voltage.

FIG. 8 illustrates a DRAM sense system where NMOST version of thepower-supply switch illustrated in FIG. 6B and the amplifier illustratedin FIG. 7A are combined with each other. As is well known, if a certainmemory cell (not shown in the drawing) within the memory-cell array isread out, for example, a negative signal voltage v_(s) will be outputtedto one of the data pair-lines (e.g., DL_(O) and /DL_(O)) which areprecharged at the voltage V_(DD)/2. Usually, this signal voltage isequal to an order of 100 mV, which is considerably small. Accordingly,this signal voltage is amplified up to V_(DD) by using a latch CMOSsense amplifier (: SA) where the well-known cross-connected NMOSamplifier and PMOS amplifier as illustrated in FIG. 7A arelongitudinally accumulated. Switching the driving MOSTs (i.e., M_(ND)and M_(PD)) ON allows the amplification to be started. Usually, however,the amplification is performed at first using the NMOS amplifier byswitching M_(ND) ON, and after that, the amplification is subsequentlyperformed using the PMOS amplifier by switching M_(PD) ON. As a result,the data line (/DL_(O) in the drawing) which is charged at the largerinitial voltage will be charged up to V_(DD), while the data line(DL_(O)) which is charged at the smaller initial voltage will bedischarged down to 0 V. More concretely, since the gates and wells ofthe MOSTs configuring the amplifiers are directly connected to eachother, as explained earlier, the signal will be effectively amplified.Namely, the threshold voltage V_(T) Of M_(N2) becomes larger.Accordingly, if M_(ND) is switched ON, the data line DL_(O) starts to bedischarged toward 0 V. After that, if M_(PD) is switched ON, V_(T) ofM_(P2) becomes smaller since DL_(O) is being discharged. Consequently,M_(P2) will charge the data line /DL_(O) even further. This acceleratesthe discharge of the data line DL_(O) by M_(N21), thereby eventuallymaking DL_(O) become equal to 0 V, and making /DL_(O) become equal toV_(DD). Also, the driving MOSTs are driven with the large voltageamplitudes. As a consequence, V_(T) at the time of the operation can bemade small enough with V_(T) at the time of the non-operation beingmaintained at constant large values. This, at the time of the operation,allows the amplifiers to be driven at high speed.

Embodiment 5

FIG. 9 illustrates a CMOS inverter which takes advantage of circuitswhere the gates and the wells are directly connected to each other.Namely, the present embodiment is a one which allows an extraordinarilylarge load capacity of device, such as data bus inside a chip, to bedriven with low power consumption and at high speed. As is well known,when driving a large load capacity, driving the large load capacity at alow voltage is advantageous from the point-of-view of the low powerconsumption. In an ordinary CMOS, however, this method results in alowering in the operation speed. In the embodiment in this drawing, theinternal main circuits are driven with an amplitude of 1 V ranging fromV_(DH) (1 V) to V_(BB) (0 V). Also, the internal large bus capacity isdriven with an amplitude of 0.5 V ranging from V_(DD) (0. 75 V) toV_(SH) (0. 25 V). The in-inverter MOSTs on the transmission side insidethe chip are driven with the large logic amplitude. As a result, even ifthe gates and the wells are directly connected to each other, thechanges in V_(T) are significantly large. This, consequently, allows thebus to be driven with the amplitude of 1 V and at high speed. Meanwhile,on the reception side inside the chip, the level conversion into thesame logic amplitude as the one of the input on the transmission side isperformed at high speed and in accordance with basically the sameoperation as the one explained in FIG. 3. Here, V_(DD) and V_(SH) areset between V_(DH) and V_(BB). Although the relative relationship amongthese voltages is the same as the one explained in FIG. 1, V_(BB) inthis embodiment is set not at the negative voltage but at the groundlevel. The reason for this is as follows: Namely, generating thenegative voltage inside the chip necessitates the use of a charge-pumpcircuit, but the charge-pump circuit lacks in its current drivingcapability. This situation makes it difficult to drive the large loadcapacity such as the data bus with the use of V_(BB) which is at thestable level. Incidentally, in this embodiment, although the directconnection between the gates and the wells is the same as in theconventional circuit, there exist the following differences: Namely, thewell voltage amplitudes are larger than the drain voltage amplitudes orsource voltage amplitudes. Otherwise, the well voltage amplitudes of thein-first-circuit MOSTs on the transmission side are different from thewell voltage amplitudes on the reception side.

Incidentally, in the double-gate MOST structure used in the embodimentsso far, the input signal is inputted into the gate of the upper MOST,and the threshold voltage V_(T) of the entire MOST is controlled usingthe control voltage inputted into the well in the lower MOST. Thesefunctions can also be made reverse. Namely, a circuit scheme is alsopossible where the input signal is inputted into the well, and where thecontrol voltage for V_(T) is inputted into the gate of the upper MOST.

According to the above-described embodiments, it becomes possible toprovide the semiconductor device whose low-voltage implementation, lowpower-consumption implementation, high-speed implementation, orsmall-size implementation has been accomplished.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A semiconductor device, comprising: a MOS transistor havingsilicon-on-insulator structure, said silicon-on-insulator structureincluding double gates and a fully-depleted silicon-on-insulator layer,said double gates being a first gate and a second gate, said second gatebeing a well layer which exists under a buried oxide film, wherein, saidfirst gate and said second gate of said MOS transistor are driven withmutually different voltage amplitudes.
 2. The semiconductor deviceaccording to claim 1, wherein said voltage amplitude for said secondgate is larger than said voltage amplitude for said first gate.
 3. Asemiconductor device, comprising: a MOS transistor havingsilicon-on-insulator structure, said silicon-on-insulator structureincluding double gates and a fully-depleted silicon-on-insulator layer,said double gates being a first gate and a second gate, said second gatebeing a well layer which exists under a buried oxide film, wherein,voltage amplitude for either of said double gates is larger than voltageamplitude for a drain or a source of said MOS transistor.
 4. Asemiconductor device wherein a power-supply switch for a block includingrepetition circuits is said MOS transistor according to claim
 1. 5. Asemiconductor device wherein a power-supply switch which is connected toeach of a plurality of sub-blocks and is selectively activated is saidMOS transistor according to claim
 1. 6. The semiconductor deviceaccording to claim 3, wherein, in said MOS transistor, said first gateand said second gate are connected to each other.
 7. A semiconductordevice, comprising: a MOS transistor having silicon-on-insulatorstructure, said silicon-on-insulator structure including double gatesand a fully-depleted silicon-on-insulator layer, said double gates beinga first gate and a second gate, said second gate being a well layerwhich exists under a buried oxide film, wherein, second-gateinsulating-film thickness is equal to an order of 1.5 nm to 2 nm in anequivalent silicon-oxide thickness.
 8. A semiconductor device wherein apower-supply switch for a block including repetition circuits is saidMOS transistor according to claim
 3. 9. A semiconductor device wherein apower-supply switch which is connected to each of a plurality ofsub-blocks and is selectively activated is said MOS transistor accordingto claim 3.